1. Field of the Invention
The present invention relates to an image decoding apparatus and image decoding method that enables a reduction in a memory bandwidth.
2. Description of the Related Art
In recent years, accompanying the spread of the MPEG (Moving Picture Experts Group) and H.264 standards and the like, image processing has become digitalized. As a result, the transmission of high quality HD image data is also being widely performed. On the other hand, because of the widespread use of portable terminals such as mobile phones, the necessity to display images of a comparatively low resolution is also high. Further, display apparatuses for displaying standard quality SD images are also in widespread use. Thus, the demand for devices that can generate images of a lower resolution (reduced image) than the resolution of the input image data is also high.
The processing speeds of most devices that are equipped with a decoding apparatus that generates a reduced image in this manner are comparatively slow. Therefore, in this kind of apparatus, there is the drawback that, in order to secure the necessary data transfer speed when performing processing to decode image data it is necessary to increase the bit width of the memory interface. For example, when the required memory bandwidth in a mobile telephone or the like is large, in order to enable storage of image data in an external memory, the number of pins of the external memory increases and the package size of the memory also increases.
In this regard, Japanese Patent Laid-Open No. 2000-4440 discloses technology that can reduce a memory bandwidth by generating an image with a reduced resolution (reduced image) when decoding high quality encoded image data. In that proposed technology, by simply compressing and retaining reference images used for decoding processing, the data amount required for decoding processing is reduced to enable reduction of the memory bandwidth.
However, according to the technology in that proposal, all the reference images are obtained by thinning-out processing, and there is thus a drawback that errors when performing difference decoding are accumulated, which deteriorates the image quality.